This invention relates to the use of genetic engineering techniques in the modification of plants. More particularly, it concerns introduction and integration of a chimeric gene coding for a polypeptide toxin produced by Bacillus thuringiensis or having substantial sequence homology to a toxin gene described below in plant cells and obtaining an insect controlling level of expression of said polypeptide toxin intracellularly by transformed plant cells and their progeny.
Recombinant DNA technology is currently used to genetically engineer certain microorganisms such as bacteria and yeast to synthesize specific proteins. Genetic engineering of higher organisms within the present state of technology requires that one or a few cells be genetically engineered from which the entire organisms can develop. Among higher organisms, the cells of certain plants exhibit excellent regeneration capability and therefore are considered potentially good material for the genetic engineering of such plants. Furthermore, in higher plants, a known system is available to introduce foreign DNA into the plant genome. This system is provided by the tumor inducing plasmid from the gram negative soil bacterium Agrobacterium tumefaciens. Agrobacterium can genetically transform plant cells by stably integrating T-DNA, a well defined fragment of the Ti plasmid, into the plant cell genome. Recently, important progress has been made to facilitate the use of the Ti plasmid as a vector for plant genetic engineering. Small directly repeated sequences which flank the T-DNA (Border sequences) have been found to play a key role in the T-DNA integration. Nononcogenic Ti plasmid vectors have been constructed from which oncogenic tumor genes have been removed by an internal deletion in the T-DNA. These Ti plasmids still contain the border sequences and consequently transfer T-DNA without tumor induction. An example of such a Ti plasmid derived vector from plant genetic engineering is pGV3850 which contains a substitution of the internal T-DNA gene by the commonly used cloning vehicle pBR322. Several procedures have been developed to regenerate infected plants which contain the pGV3850. pGV3850 with the pBR322 sequences present in its T-DNA is an efficient acceptor plasmid for gene transfer experiments in plant cells. Indeed, genes cloned in pBR322 like plasmids are transferred to Agrobacterium and inserted via homologous recombination into the pGV3850 T-DNA in a single experimental step.
Another major advance in the development of plant engineering technique is the use of plant regulatory sequences to express chimeric genes in plants. In general, these chimeric genes contain a promoter region derived from a gene which is naturally expressed in plant cells, the sequence to be expressed, and preferentially a 3' non-translated region containing a polyadenylation site of a gene which is naturally expressed in plant cells. For example, using the nopaline synthase promoter and bacterial antibiotic resistance genes, dominant selectable markers for plant cells have been constructed.
Although certain chimeric genes have now successfully been expressed in transformed plant cells, such expression is by no means straightforward. Various lines of evidence indicate that the level of expression of the foreign genes of non-plant origin not only varies greatly in different transformed tissues but are in general very low. Such low levels of gene expression could be due to several reasons: first, incomplete transcription of the gene resulting from inadvertent transcription termination signals; second, inefficient processing of the messenger RNA; third, impaired transport of the messenger RNA from the nucleus to the cytoplasm; fourth, instability of the cytoplasm messenger RNA; fifth, inefficient translation of the cytoplasm messenger RNA; and sixth, instability of the protein due to its susceptibility to plant specific proteins. Consequently, the successful transformation of plant cells using vectors such as those described above is not necessarily predictable prior to attempting a desired transformation.
Engineering of differentiated plant cells and their progeny to express the Bt2 polypeptide and/or a truncated version thereof and/or a polypeptide having substantial sequence homology thereto is far more difficult than other genes such as antibiotic resistance genes or other plant genes such as thaumatin due to one or more of the following: (1) the large size of the Bt2 toxin, even in its truncated form; (2) the particular properties of the Bt2 polypeptide (such as, but not limited to, solubility of the polypeptide); (3) the potential toxicity of the Bt2 polypeptide toward the plant cells; or (4) the Bt2 polypeptide synthesized in plant cells and their progeny must retain substantially the same properties as the crystal protein synthesized in bacteria.
Bacillus thuringiensis (referred to at times herein as B.t.) bacteria includes approximately 19 known varieties that produce polypeptide toxins which form parasporal crystals during sporulation. The crystal protein made by B.t. is toxic to the larvae of certain insects. The toxins produced by a particular variety exhibit strong insecticidal activity, against certain Lepidoptera and/or Ceoleoptera and/or Diptera larva. See e.g., Tyrell D. J. et al., J. Bacteriology, (81) 145 (No. 2): p. 1052-1062. When ingested by insect larvae, the crystals are solubilized and processed in the insect midgut to yield at least one active polypeptide toxin which is believed to act on the midgut cell membrane. Studies have revealed that individual crystal polypeptides exhibit insecticidal activity. Yamamoto, T. et al., Current Microbiology, (83) 9: p. 279-284; Yamamoto, T. et al., Arch. Biochem. Biophysics, (83) 227: (No. 1): p. 233-241; Lilley, M. et al., J. Gen. Microbiol., (80) 118: p. 1-11; Bulla, L. A. et al., J. Biol. Chem., (81) 256 (No. 6): p. 3000-3004.
The toxic activity of the crystal polypeptide produced by Bacillus thuringiensis varieties is highly specific to particular insect species and is recognized as safe to higher vertebrates.
Preparations containing the crystals are used commercially as a biological insecticide. For example: Bactospeine, distributed by Biochem Products Ltd., Dipel Abbott Laboratories; and Thuricide, Sandoz AG. The efficacy of preparations obtained from bacterial hosts is, however, limited as adequate control of pests requires repeated and precisely timed applications. In addition, costs associated with the production of such preparations have made it difficult for them to compete effectively with other commercially available products, such as pyrethoid derivatives.
Molecular genetics studies have demonstrated that at least some polypeptide toxins produced by Bacillus thuringiensis are encoded by plasmids. Stahly, D. P. et al., (1978), Biochem. Biophys. Res. Commun., 84, p. 581-588; Debaboc, V. G. et al., (1977), Genetika, 13, p. 496-501. Genes encoding toxic crystal polypeptides from different B.t. strains have been cloned and expressed in other bacterial hosts. (Schnepf & Whiteley, PNAS (81) 78:2993-2897. Klier, A. et al., EMBO J. (82) 1 (No. 7), p. 791-799; Adang et al., Gene, (36), p. 289, 1985; Schnepf et al., J. Biol. Chem., (20), p. 6264, 1985; Shibano et al., Gene, (34), 1985.
Considering the major importance of plants both for consumption and for production of valuable products, it would be highly desirable to genetically modify plants such that plant cells could synthesize polypeptide toxins substantially similar to those toxins produced by Bacillus thuringiensis, without adverse effects to the plants. By stably integrating exogenous DNA fragments coding for polypeptide toxins produced by Bacillus thuringiensis into the plant cell genome and obtaining an insect controlling level of expression of said exogenous DNA fragments in plants, plant cells and their progeny so transformed would thereby become resistant to certain insect pests. Plant cells and their progeny genetically engineered in this way would provide an economically advantageous substitute to existing commercial varieties by substantially obviating the need for specific chemical or biological insecticides, and provide a more reliable means of controlling particular insect pests, while retaining normal morphological characteristics.
It is one object of this invention to provide novel chimeric genes coding for the polypeptide toxin produced by Bacillus thuringiensis, or coding for a polypeptide toxin having substantial sequence homology to a toxin gene described herein. The chimeric genes' plant regulatory sequences direct expression in transformed plant cells.
Another object of present invention is to provide novel hybrid plasmid vectors containing said chimeric genes that allow the introduction and integration and expression of said chimeric genes in a plant cell genome.
A further object of the present invention is to provide a process for preparing genetically transformed plant cells comprising the transformation of plant cells with said hybrid plasmid vectors containing said chimeric genes.
Other objectives, features and advantages of the present invention will become apparent to those skilled in the art from the following description taken in conjunction with the accompanying drawings.